This invention relates generally to medical imaging systems, and more particularly, to methods and systems for controlling the positioning of imaging portions of medical imaging systems.
Medical imaging systems are used to perform different types of diagnostic imaging. For example, diagnostic nuclear imaging is used to study radionuclide distribution in a subject, such as a patient. Typically, one or more radiopharmaceuticals or radioisotopes are injected into the subject. Gamma camera detector heads, typically including a collimator, are placed adjacent to a surface of the subject to monitor and record emitted radiation. At least some known gamma camera detector heads are rotated around the subject to monitor the emitted radiation from a plurality of directions. The monitored radiation data from the plurality of directions is reconstructed into a three dimensional image representation of the radiopharmaceutical distribution within the subject. This three-dimensional imaging is known as Single Photon Emission Computed Tomography (SPECT).
Generally, the resolution of a gamma camera degrades with increasing distance between the imaged subject, and in particular, the imaged organ and the detector. Therefore, it is desirable to place the gamma camera as close as possible to the subject to facilitate minimizing the loss of resolution. At least some known imaging systems use non-circular orbits, such as oval or elliptical orbits to facilitate maintaining the detectors positioned close to the patient during a scan. However, this type of imaging still does not always provide optimal or close scanning and image resolution degrades.
When the imaging system is configured, for example, with a pair of gamma cameras at substantially 90 degrees to each other in what is commonly known as L-mode, which is done when imaging the heart, and other organs, the gamma cameras are configured such that the gamma cameras essentially contact one another along adjacent edges. Typical gamma cameras comprise a large scintillation crystal of NaI optically coupled to an array of Photo-Multiplying Tubes (PMTs). Signals from the array of PMTs are processed to yield the location of the scintillation event on the crystal in what is known as “Anger” camera. Because of this construction, the gamma camera is less responsive near an outer periphery of the detector. The gamma camera detector is typically sized larger than the viewing area, and a volume of missing data results in the area proximate a surface of each detector where the volume is “seen” from only one of the detectors. Generally, the body of the patient is maintained spaced away from the surface of the detectors to avoid “missing data” that causes artifacts in the reconstructed image.
It is also known to use proximity sensors in order to position the gamma cameras, for example, close to a subject being imaged. An exemplary proximity sensor arrangement is illustrated in U.S Pat. No. 5,486,700. However, in the L-mode of operation, the proximity sensors can cast shadows during imaging that also results in missing data because, for example, the patient is positioned farther away from one of the gamma cameras than the other gamma camera. In a different camera configuration, wherein the two nuclear detector heads are substantially parallel to each other in what is commonly known as H-mode, a patient table (that is detected by one of the proximity sensors) often causes the patient to be positioned further away from one of the gamma cameras than the other gamma camera. Further, because multiple proximity sensors that are typically expensive are used, for example, on each of opposite ends of the gamma cameras, the overall cost of the imaging system also increases.
In operation, some types of imaging procedures yield better performance in H-mode while others yield better performance in L-mode. For example, bone SPECT is usually performed in H-mode, which cardiac imaging is routinely performed in L-mode. Accordingly, some dual head gamma camera systems are constructed in a fixed H-mode, some in a fixed L-mode and some having a flexible configuration in which at least one camera head cane be rotated relative to the other camera head such that the system can be configured in both the L-mode and H-mode. In any mode of operation, the image quality obtained by a gamma camera is severely degraded as the distance between the detector head and the patient increases.